Because the compounds F124 and F125 can be employed as substitutes for perchlorofluorocarbons (CFC) in the field of aerosols (propellant agents) and in that of refrigeration, efficient processes for their industrial production are being investigated at present.
U.S. Pat. No. 4,766,260 describes a process for the synthesis of the compounds F123 and F124 by gas phase hydrofluorination of perhalogenated olefins, the objective being to minimize the formation of F125. Example 13 (column 6) describes the fluorination of tetrachloroethylene with a CrCl.sub.3 /Al.sub.2 O.sub.3 catalyst; despite a temperature of 350.degree. C., a long contact time (60 seconds) and a high HF/C.sub.2 Cl.sub.4 molar ratio (6/1), selectivities for F124 and F125 are low (33.3% and 7.2% respectively).
The use of a chromium(III)-based catalyst (CrCl.sub.3) supported on charcoal for the gas phase catalytic fluorination of halogenated olefins forms the subject of Japanese patent application published under No. 48-72,105/73 in which Example 4 describes the fluorination of tetrachloroethylene. Here too, despite a reaction temperature of 400.degree. C. and a high HF/C.sub.2 Cl.sub.4 molar ratio (5/1), the composition of the products formed is limited to F121 (CHCl.sub.2 -CFCl.sub.2 :6.8%), to F122 (CHCl.sub.2 -CClF.sub.2 :10.5%) and to F123 (82.7%).
U.S. Pat. No. 3,258,500 describes the use of bulk or alumina-supported chromium for gas phase catalytic fluorination reactions. In particular, Example 17 (column 14) describes the fluorination of tetrachloroethylene. At 400.degree. C. with an HF/C.sub.2 Cl.sub.4 molar ratio of 6.2/1, the selectivity for F123+F124+F125 is low (47.7%); a decrease in the reaction temperature (300.degree. C.) improves this selectivity (79.7%), but the distribution is then shifted towards less-fluorinated products (F123 and F124).
European patent application no. 349,298 describes the synthesis of the compounds F123 and F124 from pentahaloethanes by gas phase catalytic fluorination over a catalyst composed of a metal chosen from chromium, cobalt, nickel and manganese and deposited on alumina. This document places the emphasis, on the one hand, on the extensive activation of the catalyst with hydrofluoric acid (at least 90% of the support in the form of AlF.sub.3 after activation) and, on the other hand, the minimized formation of F125 during the reaction. Thus, in Example 6, which describes the fluorination of F122 at 350.degree. C. and with a long contact time (30 seconds), the selectivity for F125 is only 1.1% and the cumulative selectivity (F123+F124+F125) is only 71.5%. In Example 5, which describes the gas phase fluorination of F123 over a NiCl.sub.2 /Al.sub.2 O.sub.3 catalyst at 400.degree. C., with a contact time of 30 seconds and an HF/F123 molar ratio of 4, the selectivity for F125 is only 7.5%.
A process for producing fluorinated aliphatic hydrocarbons, based on the gas phase fluorination with hydrofluoric acid of halogenated aliphatic hydrocarbons containing at least one halogen atom other than fluorine, forms the subject of U.S. Pat. No. 3,755,477, where the catalyst is a bulk chromium oxide treated with steam before calcination and activation with hydrofluoric acid. Example 25 employs such a catalyst for the fluorination of F123 at 390.degree. C. with a high HF/F123 molar ratio (9.5/1). The selectivities for F125 and F124 are 67 and 21% respectively, but a selectivity of 2.5% for chloropentafluoroethane (F115), which cannot be recycled is also observed.
From the examination of the state-of-the-art it appears difficult to synthesize the two desired compounds (F124 and F125) with a good selectivity and a high production efficiency by direct fluorination of tetrachloroethylene or of F122. When starting with tetrachloroethylene and despite long contact times and high temperatures and molar ratios it appears difficult to obtain F124 and more especially F125 in high yields. The synthesis of both these compounds is easier from F122, but a selectivity problem (extensive formation of by-products) is confronted in this case.
As for U.S. Pat. No. 3,755,477, this shows that when starting with F123 the synthesis of F125 requires a high molar ratio (9.5) and a high temperature (390.degree. C.), the association of which results in a significant undesirable formation of F115.
In view of the interest in compounds F124 and F125 as CFC substitutes, their industrial manufacture requires a particularly efficient process, that is to say one making it possible to obtain:
a very high selectivity for F124 +F125 PA1 a high production efficiency for F124 and/or F125 PA1 a high flexibility in order to direct the manufacture at will towards the preponderant production of F124 or towards that of F125, while minimizing the by-product formation.